For many commercial, industrial, and even residential environments, power system reliability is of utmost importance. In some manufacturing or textile environments, a power outage can result in the loss of substantial quantities of product that were in the production process when the outage occurred. Further, power outages can result in down time for a facility, not only during the outage, but also due to production restarting procedures that must be undertaken subsequent to an outage. Losses of product and down time can also lead to substantial monetary losses for a facility as a result of the power outage. As such, facilities often take measures to improve or maximize power system reliability to avoid such losses.
One manner of improving power system reliability is to utilize an ungrounded power system. An advantage of an ungrounded power system is its ability to “ride-through” single phase-to-ground faults. In this regard, ungrounded systems have no intentional ground connections. As a result, when a single phase-to-ground fault occurs on an ungrounded power system, the voltage phasor triangle between the phases remains intact. Therefore, loads can remain in service while the system is experiencing the fault. FIG. 1 depicts the shift in the neutral point of an ungrounded power system experiencing a single phase-to-ground fault. In FIG. 1, the system is experiencing a phase-to-ground fault on A phase.
While ungrounded systems enjoy the benefit of riding-through single phase-to-ground faults, ungrounded systems also have drawbacks. For instance, the single phase-to-ground fault ride-through capability also places voltage stresses on the ungrounded system. Additionally, since fault currents on ungrounded systems can often have magnitudes similar to that of load current, locating a fault can also be problematic.
System protective devices, such as protective relays, may monitor system voltages and if voltage stresses are detected, the protective devices may determine that a fault is present on the system. Further, conventional solutions for locating a single phase-to-ground fault on an ungrounded system may include isolating individual bus-tie feeders and monitoring the phase voltages during the isolations. A variation of the phase voltages, such as the zero-sequence voltage, such that the voltages return to pre-fault levels may indicate the location of the fault. In radial systems, such an isolation procedure can be performed quickly, such that equipment may not be effected by the brief outage. Also, since fault current may flow in many directions relative to various sources on the system, many protective devices include a directional element for assisting in determining the location of the fault. Further, other solutions for determining the location of a phase-to-ground fault include monitoring the zero-sequence currents.
As described above, an ungrounded power system can remain operational while experiencing a single phase-to-ground fault. However, there are circumstances where conventional protective relays de-energize a large portion of a distribution system when a second, single phase-to-ground fault occurs on another phase. The existence of such a condition on the power system is a phase-to-phase-to-ground fault or a double phase-to-ground fault. Ungrounded systems may not operate properly when a double phase-to-ground fault is present on the system because two phases are electrically connected, collapsing the voltage phasor triangle. Therefore, no ride-through capability is available for double phase-to-ground faults. In these situations, differential current protection devices send trip signals to breakers across the affected zones of the power system de-energizing these zones as a means of protecting the system.
For example, consider the radial multi-source system of FIG. 2a and the ring bus configuration of FIG. 2b. Generator 1 provides a first source and generator 2 provides a second source to the power systems of FIGS. 2a and 2b. At the occurrence of a first phase-to-ground fault on A phase between buses 1 and 2, continued power service is provided due to the systems being ungrounded power systems with a single phase-to-ground fault. The A phase-to-ground fault alone on the feeder between bus 1 and bus 2 will not result in the tripping (i.e., opening) of circuit breakers on the ungrounded power systems. The same would be true if a single B phase-to-ground fault occurred between buses 3 and 4, in the absence of the depicted A phase-to-ground fault.
However, if the B phase-to-ground occurs before the A phase-to-ground fault is cleared, (i.e., isolated or repaired) a double phase-to-ground fault is detected on the system. Protective devices immediately react to isolate the faulted zones from the system. For the dual-source configuration shown in FIG. 2a, the relays associated with the feeders between bus 1 and bus 2, bus 2 and bus 3, and bus 3 and bus 4 will send trip signals to their respective breakers resulting in a differential element trip for a phase-to-phase fault or a double phase-to-ground fault. As a result, power transformers associated with bus 2 and bus 3 will have no power source.
A similar result occurs for the ring bus system of FIG. 2b, however, a protective device “racing” condition may occur due to the ring bus configuration where many protective devices may attempt to react. In the ring bus configuration case, it is possible that more of the system may be shutdown, i.e., it is possible that, in addition to the protective devices associated with the feeders between buses 1 to 2, 2 to 3, and 3 to 4 sending a trip signal to their associated breakers, the protective devices associated with the feeder between buses 4 and G2 may also send a trip signal to their respective breakers. As a result, the power transformers associated with bus 2 and bus 3 will have no power source. Additionally, in some instances, bus 4 may not have a power source and bus G2 may be taken offline.
Under conventional relaying schemes, both configurations are left with substantial portions of the systems with no power source. Accordingly, it would be desirable to develop and implement apparatuses and/or relaying schemes that provide system protection in these and other situations in a manner that maintains service to the entire system or maximizes service to the equipment on the system. In particular, it would be desirable to develop and implement apparatuses and/or relaying schemes that would provide service to the all the power transformers when two, single phase-to-ground faults occur simultaneously at different locations on a system.